Radio frequency transparent photovaltic cell

ABSTRACT

A radio frequency transparent photovoltaic cell includes a back contact layer formed of an electrically conductive material, at least one aperture formed in the back contact layer, and at least one photovoltaic cell section disposed on the back contact layer. An airship includes one or more radio frequency antennas disposed in an interior of the airship. One or more radio frequency transparent photovoltaic cells are disposed on an outer surface of the airship.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional application of U.S. application Ser.No. 12/661,859, filed on Mar. 25, 2010, the disclosure of which isincorporated by reference herein in its entirety.

FIELD

This invention relates generally to photovoltaic cells, and morespecifically to a photovoltaic cell that is transparent to radiofrequency electromagnetic radiation.

BACKGROUND

Photovoltaic cells (“PV cells”) convert light, such as sunlight, into anelectric current. PV cells may provide electric power to a system usingsolely light as an energy source.

Consequently, PV cells may provide electric power to a system withoutthe use of traditional energy sources such as chemical energy sources(e.g., batteries, fossil fuels) and/or an electric power grid.

The ability of PV cells to convert light into an electric current makesPV cells an attractive electric power source in many applicationswherein it is undesirable, impractical, uneconomical and/or impossibleto use traditional energy sources. For example, PV cells commonly poweremergency roadside call boxes located in rural areas where an electricpower grid is not readily accessible. As another example, hand heldcalculators are commonly powered by one or more PV cells; the PV cellsallow use of the calculator wherever light is present without requiringthe use of batteries or a connection to an electrical outlet.

In certain applications, it is desirable that PV cells be at leastpartially transparent to one or more predetermined frequencies of radiofrequency electromagnetic radiation. For example, if PV cells are topower a radar unit from sunlight, it may be desirable to place the PVcells on an outer surface of the radar unit so that the radar unit'sstructure will not shadow the PV cells. However, prior art PV cells aregenerally opaque to radio frequency electromagnetic radiation andtherefore, may interfere with operation of the radar unit if placed onthe radar unit's outer surface. This opaqueness is largely due to thefact that prior art PV cells generally contain at least one layer ofmetal, which blocks the passage of radio frequency electromagneticradiation through the PV cell.

A common source of metal in a PV cell is the PV cell's back contactlayer. The back contact layer is one of two electrical contactsubsystems in the PV cell that enable it to be electrically connected toan external system. Stated in another manner, the back contact layer mayserve as one of two electrical terminals on the PV cell, much like aterminal on a battery. The back contact layer functions to collectelectrical charge from the back section of the PV cell. Consequently,the back contact layer must be an electrical conductor and must be inelectrical contact with the PV cell.

As its name implies, the back contact layer is generally located nearthe back or bottom of the PV cell. The back of the PV cell is the PVcell's side that is opposite from the PV cell's light source; the frontor top of the PV cell is the PV cell's side that is proximate to thelight source. The PV cell's other electrical contact subsystem is oftenreferred to as the top contact layer or subsystem and is commonlylocated near the front of the PV cell.

A back contact layer may be formed of a material that is non-transparentto light, such as molybdenum, aluminum or nickel. Such materials arealso non-transparent to radio frequency electromagnetic radiation. Aback contact layer also may be formed of a material that is transparentto light, such as zinc oxide or indium tin oxide. Although suchmaterials are transparent to light, they are non-transparent to radiofrequency electromagnetic radiation.

Many prior art PV cells have a solid metallic back contact layer. Forexample, a prior art PV cell may have a solid back contact layer formedof or containing molybdenum. Although such cells may exhibit relativelyhigh efficiencies, the solid metallic back contact layer also blocks thepassage of radio frequency electromagnetic radiation through the PVcell.

Other prior art PV cells have a non-solid metallic back contact layer,an example of which is disclosed in U.S. Pat. No. 4,487,989 to Wakefieldet al., entitled “Contact for Solar Cell.” Wakefield discloses a PV cellwith a back contact layer having a “checkerboard pattern” intended topermit the “passage of radiation through the cell which would otherwisedecrease efficiency due to heat generation.” Although Wakefield's backcontact layer apparently passes at least some infrared radiation, whichmay generate heat, the back contact layer is not transparent to radiofrequency electromagnetic radiation of one or more predeterminedfrequencies.

Thus, there is a need for a PV cell having a back contact layer that isat least partially transparent to radio frequency electromagneticradiation of one or more predetermined frequencies (“radio frequencytransparent PV cell”).

SUMMARY

The radio frequency transparent PV cell and applications thereof hereindisclosed advance the art and may overcome at least one of the problemsarticulated above by providing a photovoltaic cell that is at leastpartially transparent to radio frequency electromagnetic radiationhaving one or more predetermined frequencies.

In particular, and by way of example only, a radio frequency transparentphotovoltaic cell includes a back contact layer formed of anelectrically conductive material, at least one aperture formed in theback contact layer, wherein each aperture has at least two elements, andat least one photovoltaic cell section disposed on the back contactlayer.

According to another embodiment, a radio frequency transparentphotovoltaic cell includes a back contact layer formed of anelectrically conductive material. At least one first aperture having afirst length is formed in the back contact layer. At least one secondaperture having a second length is formed in the back contact layer. Thefirst length is at least five (5) times the second length. At least onephotovoltaic cell section is disposed on the back contact layer.

In yet another embodiment, a method of fabricating a photovoltaic cellback contact layer that is at least partially transparent to radiofrequency electromagnetic radiation having at least one predeterminedfrequency includes creating an electrically conductive back contactlayer. The back contact layer has a thickness of at least one skin depthat a lowest predetermined frequency. One or more apertures are createdin the back contact layer, wherein each aperture is at least partiallytransparent to radio frequency electromagnetic radiation having apredetermined frequency.

In yet another embodiment, an airship includes one or more radiofrequency antennas disposed in an interior of the airship. One or moreradio frequency transparent photovoltaic cells are disposed on an outersurface of the airship.

In yet another embodiment, a method of providing electric power to anairship having one or more radio frequency antennas disposed in aninterior of the airship includes disposing one or more radio frequencytransparent photovoltaic cells on an outer surface of the airship. Eachphotovoltaic cell is at least partially transparent to radio frequencyelectromagnetic radiation having at least one predetermined frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a radio frequency transparent PVcell, according to an embodiment.

FIG. 2 is a side plan view of an application of a radio frequencytransparent PV cell, according to an embodiment.

FIG. 3 is a top plan view of a radio frequency transparent back contactlayer, according to an embodiment.

FIG. 4 is a top plan view of a radio frequency transparent back contactlayer, according to an embodiment.

FIG. 5 is a top plan view of a radio frequency transparent back contactlayer, according to an embodiment.

FIG. 6 is a cross sectional view of a radio frequency transparent PVcell, according to an embodiment.

FIG. 7 is a top plan view of a top contact subsystem, according to anembodiment.

FIG. 8 is a cross sectional view of a module of radio frequencytransparent PV cells, according to an embodiment.

FIG. 9 is a top plan view of a radio frequency transparent back contactlayer, according to an embodiment.

FIG. 10 is a top plan view of a radio frequency transparent back contactlayer, according to an embodiment.

FIG. 11 is a cross sectional view of a module of radio frequencytransparent PV cells, according to an embodiment.

FIG. 12 is a cross sectional view of a module of radio frequencytransparent PV cells, according to an embodiment.

FIG. 13 is a side plan view of a prior art airship.

FIG. 14 is a side plan view of an airship having a plurality of radiofrequency transparent PV cells, according to an embodiment.

DETAILED DESCRIPTION

The present teaching is by way of example only, not by way oflimitation. The concepts herein are not limited to use or applicationwith a specific type of radio frequency transparent PV cell. Thus,although the instrumentalities described herein are for the convenienceof explanation, shown and described with respect to exemplaryembodiments, it will be appreciated that the principles herein may beapplied equally in other types of radio frequency transparent PV cells.

FIG. 1 is a cross sectional view of radio frequency transparentphotovoltaic cell 100, also referred to as PV cell 100. PV cell 100generates an electric current (not shown) in response to incident light110. PV cell 100 has top or front surface 108 which receives incidentlight 110. Bottom or back surface 112 is opposite of top or frontsurface 108.

PV cell 100 includes back contact layer 102 having thickness 106. Backcontact layer 102 is formed of or contains a metallic material. Backcontact layer 102 has at least one aperture (not shown in FIG. 1); theat least one aperture provides a means for passing radio frequencyelectromagnetic radiation of one or more predetermined frequencies.Consequently, back contact layer 102 is at least partially transparentto one or more predetermined frequencies of radio frequencyelectromagnetic radiation. Back contact layer 102 is discussed in moredetail below with respect to FIGS. 3-5 and 9-10.

At least one photovoltaic cell section 104 is disposed on back contactlayer 102; a plurality of photovoltaic cell sections 104 may be disposedon back contact layer 102 to create a module of a plurality ofmonolithically integrated PV cells. Each photovoltaic cell section 104is at least partially transparent to the one or more predeterminedfrequencies of electromagnetic radiation. Photovoltaic cell section 104is generally relatively thin and often contains little metal; thereforephotovoltaic cell section 104 is often inherently transparent to radiofrequency electromagnetic radiation. However, as stated above, backcontact layer 102 is formed of or contains a metallic material, andmetallic materials inherently block the passage of radio frequencyelectromagnetic radiation. Consequently, the properties of back contactlayer 102 often primarily determine whether PV cell 100 is transparentto radio frequency electromagnetic radiation.

PV cell 100 may be constructed according to PV cell technologies knownin the art. For example, PV cell 100 may be a homojunction device, aheterojunction device, a p-i-n device, an n-i-p device, or amultijunction device. In embodiments where PV cell 100 is amultijunction device, PV cell 100 may have a plurality of back contactlayers 102, and/or PV cell sections 104. In some embodiments, PV cell100 is a thin film heterojunction device; exemplary thin filmheterojunction embodiments are discussed below with respect to FIGS. 6and 11-12.

FIG. 2 illustrates an application of PV cell 100, wherein PV cell 100 isdisposed between light source 204 (e.g., the sun) and antenna 202. Frontsurface 108 receives light 210, and back surface 112 is proximate toantenna 202. In the application of FIG. 2, it is desired that PV cell100 generate an electric current in response to light 210. However,because PV cell 100 is proximate to antenna 202, it is also desired thatPV cell 100 be transparent to microwave energy 206 generated by antenna202. Microwave energy 206 has a predetermined frequency. Accordingly,back contact layer 102 is tuned to pass electromagnetic radiation of thepredetermined frequency. Consequently, as illustrated in FIG. 2,microwave energy 206 impinges back surface 112, passes through backcontact layer 102 and PV cell section 104, and emerges from frontsurface 108 as microwave energy 208.

FIG. 3 is a top plan view of back contact layer 300. Back contact layer300, which is tuned to be at least partially transparent to radiofrequency electromagnetic radiation of a predetermined frequency, is anembodiment of back contact layer 102.

Back contact layer 300 is formed of electrically conductive metallicmaterial 302. In an embodiment, material 302 is formed of or containsmolybdenum. Material 302 has thickness 106 (shown in FIG. 1) at leastequal to one skin depth (8) at the predetermined frequency. Preferably,material 302 has thickness 106 of at least two skin depths. Skin depthis the distance an electromagnetic wave that strikes and penetratesmaterial 302 travels within material 302 until the wave's magnitude isattenuated to 1/e of its original magnitude. The constant e is the baseof the natural logarithm and is also known as Napier's constant. Theconstant e is approximately equal to 2.7183. Skin depth is dependent onthe frequency of the impinging electromagnetic wave as well asproperties of material 302. Skin depth may be determined using Equation1 as follows:

$\begin{matrix}{\delta = {\sqrt{\frac{\rho}{\pi \; f\; \mu}}{meters}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

In Equation 1, ρ is the electrical resistivity in ohms-meters ofmaterial 302, μ is the permeability in henries per meter of material302, and f is the frequency of the electromagnetic radiation impingingmaterial 302.

As an example, if back contact layer 300 is to be tuned to be at leastpartially transparent to radio frequency electromagnetic radiationhaving a predetermined frequency of 10 GHz, and material 302 consists ofmolybdenum, the skin depth of material 302 at the predeterminedfrequency of 10 GHz may be determined as follows. First, the resistivityand permeability of molybdenum are determined. The resistivity ofmolybdenum is about 5.7×10⁻⁸ ohms-meter, and the permeability ofmolybdenum is about 4π×10⁻⁷ henries per meter. Using Equation 1, theskin depth of molybdenum at 10 GHz may be calculated as follows:

$\begin{matrix}{\delta = {\sqrt{\frac{5.7 \times 10^{- 8}}{{\pi \left( 10^{10} \right)}\left( {4\; \pi \; 10^{- 7}} \right)}} = {1.20 \times 10^{- 6}{{{meters}\left( {1.20\mspace{14mu} {microns}} \right)}.}}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

Because the skin depth is 1.20 microns, material 302 has thickness 106of at least 1.20 microns.

Back contact layer 300 has apertures 304 which allow electromagneticradiation of a predetermined frequency to at least partially passthrough back contact layer 300. Although back contact layer 300 is shownhaving four (4) apertures 304, back contact layer 300 may have a greateror smaller quantity of apertures 304. In an embodiment, at least ninety(90) percent of a surface area of back contact layer 300 is electricallyconductive and thus not covered with apertures 304.

The geometric form (e.g., shape and size) of apertures 304 determinewhat frequency of radio frequency electromagnetic radiation apertures304 will pass through back contact layer 300. Apertures 304 have a crossshape; each aperture 304 has horizontal element 318 and vertical element320. Horizontal element 318 and vertical element 320 are displaced fromeach other at about ninety (90) degrees. Apertures 304 are essentiallysymmetrical; dimension 322 is about equal to dimension 324, anddimension 326 is about equal to dimension 328.

The frequency of electromagnetic radiation that may pass throughapertures 304 is largely dependent on the lengths of each element,lengths 306 and 308. The length of each element is about equal to onehalf of the wavelength of the electromagnetic radiation at thepredetermined frequency. The length of each element (lengths 306 and308) may be approximated as a function of the predetermined frequency(f) as follows:

$\begin{matrix}{{length} \approx {\frac{3 \times 10^{8}}{2\; f}{{meters}.}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

As an example, if back contact layer 300 is to be at least partiallytransparent to electromagnetic radiation having a predeterminedfrequency of ten (10) GHz, lengths 306 and 308 of the elements 318 and320 respectively are approximated as follows using Equation 3:

$\begin{matrix}{{{{length} \approx \frac{3 \times 10^{8}}{2\left( {10 \times 10^{9}} \right)}} = {1.5\mspace{14mu} {centimeters}\; {\left( {15\mspace{11mu} {millimeters}} \right).}}}\;} & {{Equation}\mspace{14mu} 4}\end{matrix}$

Consequently, lengths 306 and 308 would be approximately 15 millimeters.

The bandwidth of each aperture 304 is dependent on widths 310 and 312 ofeach element, as well as on spacing 314 and 316 of each aperture.Bandwidth is the size of the frequency spectrum, which is centered aboutthe predetermined frequency, that apertures 304 are transparent to. Inorder to determine the bandwidth of an aperture, a maximum permittedattenuation level must be defined. For example, if the maximum permittedattenuation level is defined as ten (10) dB, then an aperture isconsidered to pass electromagnetic radiation only to the extent theaperture attenuates the electromagnetic radiation by ten (10) dB orless. Thus, if an aperture has a bandwidth of two hundred (200) MHz, theaperture allows electromagnetic radiation having a frequency within onehundred (100) MHz of the predetermined frequency to pass through theaperture with ten (10) dB or less of attenuation.

The bandwidth of apertures 304 generally becomes smaller as the ratiosof dimensions 314 to 306 and dimensions 316 to 308 are increased.Similarly, the bandwidth of apertures 304 also becomes smaller if theratios of dimensions 314 to 312 and dimensions 316 to 310 are increased.

Apertures 304 are non-polarized because elements 318 and 320 areessentially perpendicular to each other. A non-polarized aperture passeselectromagnetic radiation that is either perpendicular or parallel toany given side of the aperture. Apertures 304 will at least partiallypass electromagnetic radiation at the predetermined frequency that iscollinear to either elements 318 or 320. An embodiment of a back contactlayer having polarized apertures is discussed below with respect to FIG.5.

As discussed above, back contact layer 300 at least partially passesradio frequency electromagnetic radiation having a single predeterminedfrequency because back contact layer 300 has apertures of a singlegeometric form. However, a back contact layer may be formed that atleast partially passes radio frequency electromagnetic radiation of aplurality of predetermined frequencies by including a plurality ofdifferent sized and/or shaped apertures.

FIG. 4 is a top plan view of radio frequency transparent back contactlayer 400, which is an embodiment of back contact layer 102. Backcontact layer 400 may at least partially pass radio frequencyelectromagnetic radiation of two predetermined frequencies.

Back contact layer 400 is formed of or contains electrically conductivemetallic material 406. In an embodiment, material 406 is formed of orcontains molybdenum. Back contact layer 400 has thickness 106 (shown inFIG. 1) of at least one skin depth at the lowest predeterminedfrequency. Preferably, back contact layer 400 has thickness 106 of atleast two skin depths at the lowest predetermined frequency. Skin depthmay be determined using Equation 1 as discussed above with respect toFIG. 3.

Back contact layer 400 has large aperture 402 and small apertures 404.Although back contact layer 400 is illustrated having one (1) largeaperture 402 and sixteen (16) small apertures 404, back contact layer400 may have a greater or smaller quantity of each size aperture. In anembodiment, at least ninety (90) percent of a surface area of backcontact layer 400 is electrically conductive and thus not covered withapertures 402 or 404. Moreover, it is understood and appreciated that inat least one embodiment, back contact layer 400 has at least a firstaperture having a first geometric form and a second aperture having asecond geometric form. The difference between the first and secondgeometric forms may be a matter of scale, as in aperture 402 andapertures 404, or they may be entirely different in shape and form. Asshown, in at least one embodiment the first and second geometric form isthat of a cross shape.

Aperture 402 has horizontal element 408 and vertical element 410. Thelength of horizontal element 408 is about equal to the length ofvertical element 410, and horizontal element 408 is displaced fromvertical element 410 by about ninety (90) degrees.

Apertures 404 have horizontal elements 412 and vertical elements 414.The length of each horizontal element 412 is about equal to the lengthof each vertical element 414, and each horizontal element 412 isdisplaced from each vertical element 414 by about ninety (90) degrees.In an embodiment, the length of horizontal element 408 is at least five(5) times the length of horizontal element 412, and the length ofvertical element 410 is at least five (5) times the length of verticalelement 414.

Equation 3 may be arranged such that the predetermined frequency (f) ofan aperture is largely a function of its length as follows:

$\begin{matrix}{f \approx {\frac{3 \times 10^{8}}{2({length})}{{Hz}.}}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

It can be determined by inspecting Equation 5 that the predeterminedfrequency (f) is inversely proportional to aperture length.

In back contact layer 400, the lengths of elements 408 and 410 of largeaperture 402 are greater than the lengths of elements 412 and 414 ofsmall apertures 404. Consequently, apertures 404 are at least partiallytransparent to electromagnetic radiation of a higher frequency thanaperture 402. Accordingly, aperture 402 at least partially passeselectromagnetic radiation of the lower predetermined frequency, andapertures 404 at least partially pass electromagnetic radiation of thehigher transmission frequency. For example, apertures 404 may be tunedto pass X-band microwave electromagnetic radiation of a predeterminedfrequency of ten (10) GHz, and aperture 402 may be tuned to pass UHFband microwave electromagnetic radiation of a predetermined frequency offour hundred fifty (450) MHz. Thus, back contact layer 400 as a wholemay be at least partially transparent to microwave electromagneticradiation of a predetermined frequency of ten (10) GHz, as well asmicrowave electromagnetic radiation of a predetermined frequency of fourhundred fifty (450) MHz. In an embodiment, if an upper frequency rangeof radio frequency is defined as three hundred (300) GHz, lengths ofelements 408, 410, 412 and 414 are at least zero-point-five (0.5)millimeters.

FIG. 5 is a top plan view of back contact layer 500, which is anembodiment of back contact layer 102. Back contact layer 500 may be atleast partially transparent to radio frequency electromagnetic radiationof two predetermined frequencies.

Back contact layer 500 is formed of or contains electrically conductivemetallic material 510. In an embodiment, material 510 is formed of orcontains molybdenum. Back contact layer 500 has thickness 106 (shown inFIG. 1) of at least one skin depth at the lowest predeterminedfrequency. Preferably, back contact layer 500 has thickness 106 of atleast two skin depths at the lowest predetermined frequency. Skin depthmay determined using Equation 1 as discussed above with respect to FIG.3.

Back contact layer 500 includes large apertures 502 and small apertures504. Although back contact layer 500 is illustrated as having two (2)large apertures 502 and one hundred sixty-eight (168) small apertures504, back contact layer 500 can have any quantity of large apertures 502and any quantity of small apertures 504. Furthermore, back contact layer500 can have one or more apertures of one or more different sizes and/orshapes. Although large apertures 502 and small apertures 504 areillustrated as being orthogonal to each other, apertures in back contactlayer 500 may be displaced at any angle with respect to other apertures.In an embodiment, a length of large aperture 502 is at least five (5)times the length of small aperture 504. In an embodiment, if an upperfrequency range of radio frequency is defined as three hundred (300)GHz, lengths of the long sides of apertures 502 and 504 are at leastzero-point-five (0.5) millimeters. In an embodiment, at least ninety(90) percent of a surface area of back contact layer 500 is electricallyconductive.

Apertures 502 and 504 are rectangular shaped. Consequently, apertures502 and 504 are polarized, and therefore, each aperture only passeselectromagnetic radiation polarized with a magnetic field parallel to(or collinear with) the aperture.

Apertures increase the resistance of back contact layer 500 bydecreasing the conductive surface area of back contact layer 500.Consequently, the resistance of back contact layer 500 decreases asaperture surface area decreases. Because rectangular apertures, such asapertures 502 and 504, only have a single element, rectangular aperturesmay have a smaller surface area than corresponding cross shapedapertures; therefore back contact layer 500 may have a lower resistancethan a back contact layer having corresponding cross shaped apertures.In some embodiments, decreasing the resistance of a back contact layerimproves the efficiency of the PV cell constructed with the back contactlayer. Consequently, in some embodiments, a PV cell's efficiency can beimproved by constructing the PV cell with a back contact layer havingrectangular apertures instead of cross shaped apertures.

As discussed above, PV cell 100 may be a thin film heterojunctiondevice. FIG. 6 is a cross sectional view of radio frequency transparentPV cell 600, which is a thin film heterojunction device. A thin film PVcell is formed by consecutively depositing a plurality of layers on asubstrate. PV cell 600 is an embodiment of PV cell 100 and is at leastpartially transparent to radio frequency electromagnetic radiationhaving one or more predetermined frequencies. In PV cell 600, PV cellsection 104 includes solar absorber layer 604, window layer 606, topcontact layer 608, and encapsulation layer 610.

Substrate 602 serves as a base to PV cell 600. Substrate 602 iselectrically non-conductive and at least partially transparent to radiofrequency electromagnetic radiation of the one or more predeterminedfrequencies. In an embodiment, substrate 602 is flexible, which mayresult in PV cell 602 being flexible. Substrate 602 may be formed of orcontain a polyimide material.

Back contact layer 102 is disposed on substrate 602. Back contact layer102 provides an electrical connection to PV cell 600, and back contactlayer 102 is tuned to be at least partially transparent to radiofrequency electromagnetic radiation of the one or more predeterminedfrequencies. In an embodiment, back contact layer 102 is formed of orcontains molybdenum. By way of example and not of limitation, backcontact layer 102 may be back contact layer 300 of FIG. 3, back contactlayer 400 of FIG. 4, or back contact layer 500 of FIG. 5.

Photovoltaic section 104 includes four discrete layers, solar absorberlayer 604, window layer 606, top contact layer 608, and encapsulationlayer 610. Solar absorber layer 604 is deposited on back contact layer102 opposite of substrate 602. Solar absorber layer 604 absorbs light110 that impinges top surface 612 and travels through encapsulationlayer 610, top contact layer 608, and window layer 606 to reach solarabsorber layer 604. Solar absorber layer 604 also forms one half of aP/N junction. A P/N junction is a junction of two layers of dissimilarsemiconductor materials. One of the layers of semiconductor materialshas a surplus of electrons as is frequently referred to as “n-type”material; the other layer of semiconductor materials has a surplus ofholes and is frequently referred to as “p-type” material. Solar absorberlayer 604 is at least partially transparent to radio frequencyelectromagnetic radiation having the one or more predeterminedfrequencies. In an embodiment, solar absorber layer 604 is a p-typematerial formed of or including copper, indium, gallium and selenide.

Window layer 606 is disposed on solar absorber layer 604 opposite ofback contact layer 102. Window layer 606, which forms the second half ofthe P/N junction, allows incident light 110 to reach solar absorberlayer 604. Window layer 606 is at least partially transparent to radiofrequency electromagnetic radiation having the one or more predeterminedfrequencies. In an embodiment, window layer 606 is an n-type materialformed of or including cadmium sulfide.

Top contact layer 608 is disposed on window layer 606 opposite of solarabsorber layer 604. Top contact layer 608 collects electrical chargegenerated near the top of photovoltaic cell section 104. Top contactlayer 608 is at least partially transparent to light 110 to allow light110 to reach the remainder of photovoltaic cell section 104.Additionally, top contact layer 608 is at least partially transparent toradio frequency electromagnetic radiation having the one or morepredetermined frequencies. Furthermore, top contact layer 608 must beelectrically conductive so that it can collect electric charge.

FIG. 7 illustrates top contact subsystem 700, which is an embodiment oftop contact layer 608. Top contact subsystem 700 functions to collectelectric charge from an area near top 612 of PV cell 600. Top contactsubsystem 700 includes bus bar 702, which is formed of or includes ametallic material. Bus bar 702 serves to consolidate electric chargecollected by conductive fingers 704, which are also formed of or includea metallic material. Bus bar 702 may serve as one of two electricalconnection points to interface PV cell 600 to an external system. (Backcontact layer 102 may serve as the other electrical connection point.)

Fingers 704 occupy a relatively small proportion of surface 706 of topcontact subsystem 700. Consequently, a significant portion of light andradio frequency electromagnetic radiation that impinge top contactsubsystem 700 pass through top contact subsystem 700.

Returning to FIG. 6, encapsulation layer 610 is disposed on top contactlayer 608 opposite of window layer 606. Encapsulation layer 610passivates PV cell 600. Stated in another manner, encapsulation layer610 protects PV cell 600 from adverse environmental elements, such asprecipitation. In an embodiment, encapsulation layer 610 hasantireflective properties to reduce the amount of incident light 110reflected by PV cell 600. Encapsulation layer 610 is at least partiallytransparent to radio frequency electromagnetic radiation having the oneor more predetermined frequencies.

As stated above, all layers of PV cell 600 are at least partiallytransparent to radio frequency electromagnetic radiation having the oneor more predetermined frequencies. Consequently, PV cell 600 is at leastpartially transparent to electromagnetic radiation having the one ormore predetermined frequencies.

As stated above with respect to FIG. 1, a plurality of PV cell sections104 may be disposed on back contact layer 102 in order to form a moduleof PV cells. Stated in another manner, back contact layer 102 may beused as a base for a module of monolithically integrated PV cells. FIG.8 illustrates module 800, which is an embodiment of PV cell 100 having aplurality of PV cell sections 104. Module 800 includes PV cell sections802, 804 and 806, each of which are embodiment of PV cell section 104.PV cell sections 802, 804 and 806 are disposed on back contact layer808, which is an embodiment of back contact layer 102. Although FIG. 8illustrates module 800 as having three (3) PV cell sections, module 800can have any quantity of PV cell sections.

FIG. 9 is a top plan view of back contact layer 900, which is anembodiment of back contact layer 808 of FIG. 8. Back contact layer 900may be used in module 800 to create a module having a plurality of PVcells electrically connected in a series configuration.

Back contact layer 900 is formed of or contains electrically conductive,metallic material 932. In an embodiment, material 932 is formed of orcontains molybdenum. Back contact layer 900 has thickness 106 (shown inFIG. 1) of at least one skin depth at the lowest predeterminedfrequency. Preferably, back contact layer 900 has thickness 106 of atleast two skin depths at the lowest predetermined frequency. Skin depthmay determined using Equation 1 as discussed above with respect to FIG.3.

Back contact layer 900 is illustrated as having four identically sizedand shaped apertures 902. Consequently, back contact layer 900 is atleast partially transparent to electromagnetic radiation having a singlepredetermined frequency, wherein the predetermined frequency is largelydetermined by the lengths of horizontal elements 926 and verticalelements 928. However, back contact layer 900 may have any quantity ofapertures, and back contact layer 900 may have a plurality of differentsized and/or shaped apertures. Accordingly, other embodiments of backcontact layer 900 may be at least partially transparent toelectromagnetic radiation having a plurality of predeterminedfrequencies.

In an embodiment, apertures 902 are used to partially delineateindividual PV cells in module 800. However, because back contact layer900 is intended to be used to create a module having a plurality of PVcells connected in a series configuration, each individual PV cell mustbe completely delineated. Accordingly, additional cuts, or scribes,illustrated as lines 906 in FIG. 9, are made to further delineate eachPV cell. Finally, outer edge 904 is used to complete delineation of atleast some PV cells. In an embodiment, scribes 906 may be collinear withhorizontal elements 926 and vertical elements 928 of apertures 902. Inthe embodiment illustrated in FIG. 9, back contact layer 900 delineatesnine (9) individual PV cells represented as PV cells 908, 910, 912, 914,916, 918, 920, 922 and 924.

FIG. 10 is a top plan view of back contact layer 1000, which is anembodiment of back contact layer 808 of FIG. 8. Back contact layer 1000may be used in module 800 to create a module having a plurality of PVcells electrically connected in a parallel configuration.

Back contact layer 1000 is formed of or contains electrically conductivemetallic material 1006. In an embodiment, material 1006 is formed of orcontains molybdenum. Back contact layer 1000 has thickness 106 (shown inFIG. 1) of at least one skin depth at the lowest predeterminedfrequency. Preferably, back contact layer 1000 has thickness 106 of atleast two skin depths at the lowest predetermined frequency. Skin depthmay be determined using Equation 1 as discussed above with respect toFIG. 3.

Back contact layer 1000 is illustrated as having four identically sizedand shaped apertures 1002. Consequently, back contact layer 1000 is atleast partially transparent to electromagnetic radiation have a singlepredetermined frequency, wherein the frequency is largely determined bythe lengths of horizontal elements 1026 and vertical elements 1028.However, back contact layer 1000 may have any quantity of apertures, andback contact layer 1000 may have a plurality of different sized and/orshaped apertures. Accordingly, other embodiments of back contact layer1000 may be at least partially transparent to electromagnetic radiationhaving a plurality of predetermined frequencies.

In an embodiment, apertures 1002 are used to partially delineateindividual PV cells in module 800. However, because back contact layer1000 is intended to be used to create a module having a plurality of PVcells connected in a parallel configuration, the back contact layer ofeach PV cell must be electrically connected to the back contact layer ofeach other PV cell. Accordingly, each PV cell is partially delineated byapertures 1002, and some PV cells are also partially delineated by outeredge 1004. However, the back contact layers of each individual PV cellare electrically interconnected by solid back contact layer sectionscollinear with horizontal elements 1026 and vertical elements 1028. Inthe embodiment illustrated in FIG. 10, back contact layer 1000 partiallydelineates nine (9) individual PV cells represented as PV cells 1008,1010, 1012, 1014, 1016, 1018, 1020, 1022 and 1024.

FIG. 11 is a cross sectional view of module 1100 of a plurality of PVcells. Module 1100 is an embodiment of module 800 of FIG. 8. Module 1100includes back contact layer 900 of FIG. 9, and the cross section ofmodule 1100 is taken along line 930 of FIG. 9.

Module 1100 is at least partially transparent to radio frequencyelectromagnetic radiation having one or more predetermined frequencies.Although module 1100 is illustrated as having three (3) PV cells 1120,1122, and 1124, module 1100 may have any quantity of PV cells. Each PVcell in module 1100 is electrically connected in a series configuration.

Module 1100 is monolithically integrated onto substrate 1102. Substrate1102 is electrically non-conductive and at least partially transparentto radio frequency electromagnetic radiation of the one or morepredetermined frequencies. In an embodiment, substrate 1102 is flexible,and module 1100 is flexible. In an embodiment, substrate 1102 may beformed of or contain a polyimide material.

Back contact layer 900 is disposed on substrate 1102. Sections 1104 a,1104 b and 1104 c of back contact layer 900 are visible in FIG. 11.Sections 1104 a, 1104 b and 1104 c respectively correspond to sections922, 916, and 924 of back contact layer 900.

Sections 1106 a and 1106 b are non-electrically conductive. Sections1106 a and 1106 b represent cross sections of scribes 906 as illustratedin FIG. 9.

Sections 1104 a, 1104 b and 1104 c of back contact layer 900 each formthe base of PV cells 1120, 1122 and 1124, respectively. Respectivelydisposed on back contact layer sections 1104 a, 1104 b and 1104 c aresolar absorber layer sections 1108 a, 1108 b and 1108 c. Solar absorberlayer sections 1108 a, 1108 b and 1108 c form one half of a P/N junctionand absorb light 110 that reaches the solar absorber layer sections.Solar absorber layer sections 1108 a, 1108 b and 1108 c are at leastpartially transparent to radio frequency electromagnetic radiationhaving the one or more predetermined frequencies. In an embodiment,solar absorber layer sections 1108 a, 1108 b and 1108 c are a p-typematerial formed of or including copper, indium, gallium and selenide.

Respectively disposed on solar absorber layer sections 1108 a, 1108 b,and 1108 c are window layer sections 1110 a, 1110 b and 1110 c, whichform the second half of P/N junctions. Window layer sections 1110 a,1110 b and 1110 c help direct incident light 110 to solar absorber layersections 1108 a, 1108 b and 1108 c, respectively. Window layer sections1110 a, 1110 b and 1110 c are at least partially transparent to radiofrequency electromagnetic radiation having the one or more predeterminedfrequencies. In an embodiment, window layer sections 1110 a, 1110 b and1110 c are an n-type material formed of or including cadmium sulfide.

Respectively disposed on window layer sections 1110 a, 1110 b and 1110 care top contact layer sections 1112 a, 1112 b and 1112 c. Because the PVcells 1120, 1122 and 1124 are electrically connected in a seriesconfiguration, the top of PV cell 1120 is electrically connected to thebottom of PV cell 1122, and the top of PV cell 1122 is electricallyconnected to the bottom of PV cell 1124. Accordingly, connector 1126 aconnects top contact layer section 1112 a to back contact layer section1104 b, and connector 1126 b connects top contact layer section 1112 bto back contact layer section 1104 c. Connectors 1126 a and 1126 b areelectrically conductive.

Top contact layer sections 1112 a, 1112 b and 1112 c are at leastpartially transparent to light 110 to allow light 110 to reach theremainder of photovoltaic cells 1120, 1122 and 1124, respectively.Additionally, top contact layer sections 1112 a, 1112 b and 1112 c areat least partially transparent to radio frequency electromagneticradiation having the one or more predetermined frequencies. Furthermore,top contact layer sections 1112 a, 1112 b and 1112 c must beelectrically conductive so that they can collect electric charge. In anembodiment, top contact layer sections 1112 a, 1112 b and 1112 c areembodiments of top contact subsystem 700 of FIG. 7.

Insulating section 1116 a separates PV cell 1120 from PV cell 1122;insulating section 1116 b separates PV cell 1122 from PV cell 1124.Insulating sections 1116 a and 1116 b may be formed by cutting oretching module 1100, and insulating sections 1116 a and 1116 b may befilled with an insulating material.

Encapsulation layer 1114 is applied to top contact layer sections 1112a, 1112 b and 1112 c as well as to insulating sections 1116 a and 1116b. As was discussed with respect to FIG. 6, encapsulation layer 1114passivates module 1100. In an embodiment, encapsulation layer 1114 hasantireflective properties to reduce the amount of incident light 110reflected by module 1100. Encapsulation 1114 is at least partiallytransparent to radio frequency electromagnetic radiation having the oneor more predetermined frequencies.

FIG. 12 is a cross sectional view of module 1200 of a plurality of PVcells. Module 1200 is an embodiment of module 800 of FIG. 8, and issimilar to module 1100 of FIG. 11. Module 1200 includes back contactlayer 1000 of FIG. 10, and the cross section of FIG. 12 is taken alongline 1030 of FIG. 10.

Module 1200 is at least partially transparent to electromagneticradiation having one or more predetermined frequencies. Although module1200 is illustrated as having three (3) PV cells 1204, 1206 and 1208,module 1200 may have any quantity of PV cells. Each PV cell in module1200 is electrically connected in a parallel configuration.

Module 1200 is monolithically integrated onto substrate 1102, which wasdiscussed with respect to FIG. 11. Back contact layer 1000 is disposedon substrate 1102. As illustrated in FIG. 12, back contact layer 1000 isuninterrupted due to the cross section of module 1200 being taken alongline 1030 of FIG. 10. Line 1030 traverses sections of back contact layer1000 that are not interrupted by apertures 1002. As was discussed above,the back contact layer sections of each PV cell must be electricallyinterconnected if each PV is to be electrically connected in a parallelconfiguration.

Respectively disposed on back contact layer 1000 are solar absorberlayer sections 1108 a, 1108 b and 1108 c. Respectively disposed on solarabsorber layer section 1108 a, 1108 b and 1108 c are window layersections 1110 a, 1110 b and 1110 c. Solar absorber layer section 1108 a,1108 b and 1108 c and window layer sections 1110 a, 1110 b and 1110 care discussed with respect to FIG. 11.

Insulating section 1116 a separates PV cell 1204 from PV cell 1206;insulating section 1116 b separates PV cell 1206 from PV cell 1208.Insulating sections 1116 a and 1116 b may be formed by cutting oretching module 1200, and insulating sections 1116 a and 1116 b may befilled with an insulating material.

Top contact layer 1202 is disposed on window layer sections 1110 a, 1110b and 1110 c as well as insulating sections 1116 a and 1116 b. Topcontact layer 1202 electrically connects the window layer of each PVcell, as required for each PV cell to be connected in a parallelelectrical configuration. Top contact layer 1202 is at least partiallytransparent to light 110 to allow light 110 to reach the remainder of PVcells 1204, 1206 and 1208. Additionally, top contact layer 1202 is atleast partially transparent to radio frequency electromagnetic radiationhaving the one or more predetermined frequencies. Furthermore, topcontact layer 1202 must be electrically conductive so that it cancollect electric charge. In an embodiment, top contact layer 1202includes a plurality of top contact subsystems 700 of FIG. 7.

Encapsulation layer 1114 is applied to top contact layer 1202.Encapsulation layer 1114 is discussed with respect to FIG. 11.

With respect to the PV cells as discussed and described above, it isunderstood and appreciated that many different applications may presentthemselves where a radio frequency transparent photovoltaic cell orcells are advantageously desirable. For example, the operation of an RFtransceiver within a building or vehicle having the above describe PVcells deployed on the roof or outer surface for energy production.

At least one specific application of PV cells is to power high altitudeairships that remain in flight for a long period of time. Examples ofsuch airships includes a radar airship, which locates objects, and acommunication airship, which relays communication signals betweenstations. Because such airships do not frequently return to the groundfor refueling, they must have one or more energy sources that will powerthem for a long duration of time. Although a large amount of one or morechemical energy sources, such as batteries or fossil fuels, couldpossibly power such airships for a sufficiently long duration of time,the volume and/or weight of such chemical energy sources makes their useimpractical or impossible. Consequently, PV cells, which can potentiallypower airships indefinitely using solely sunlight as an energy source,are potentially an attractive energy source for airships.

Many airships, such as radar and communication airships, must transmitand/or receive radio frequency electromagnetic radiation. For example, aradar airship may detect an object by emitting radio frequencyelectromagnetic radiation, receiving returned radio frequencyelectromagnetic radiation reflected by the object, and analyzing adifference between the emitted and returned radio frequencyelectromagnetic radiation. As another example, a communication airshipmay receive data in the form of first microwave signals from a basestation, and may forward this data to remote recipients in the form ofsecond microwave signals.

The transfer and/or receipt of electromagnetic radiation requires anantenna to convert electrical signals to electromagnetic signals andvice versa. Consequently, airships that transmit and/or receiveelectromagnetic radiation, such as radar or communication airships,require at least one antenna.

An antenna may be placed on an airship's outer surface. An advantage ofsuch placement is that the airship's structure does not shadow theantenna and thereby interfere with the antenna's transmission and/orreception of electromagnetic radiation. However, placement of theantenna on the airship's outer surface decreases the surface areaavailable for placement of PV cells to power the airship. If anairship's electrical power requirements are great enough to require asignificant portion of the airship's outer surface to be covered with PVcells, placing an antenna on the airship's outer surface may not befeasible.

Alternately, an antenna may potentially be placed inside an airship.Such practice may free surface area on the airship's outer surface forplacement of PV cells. However, the antenna must be able to transmit andreceive electromagnetic radiation through the airship's outer surface.Consequently, at least a portion of the airship's outer surface must betransparent to electromagnetic radiation in the frequencies of interestif the antenna is going to be placed within the airship.

FIG. 13 is a side plan view of prior art airship 1300 having microwavefrequency antenna 1306 located within interior 1304 of prior art airship1300. Prior art airship 1300 includes prior art PV cell modules 1308disposed on outer surface 1302. Outer surface 1302 is curved, therefore,prior art PV cell modules 1308 must be flexible. Because antenna 1306must be able to transmit and/or receive radio frequency electromagneticradiation through outer surface 1302, prior art PV cell modules 1308,which are opaque to radio frequency electromagnetic radiation, arelocated solely near top 1310 of prior art airship 1300 so that part ofouter surface 1302 is at least partially transparent to radio frequencyelectromagnetic radiation.

Although prior art airship 1300 may be adequate for some airshipapplications, many modern airship applications require relatively largeamounts of electrical power to power systems within the airship.Unfortunately, PV cells generally have relatively low efficiencies;consequently, a large surface area of PV cells is generally required togenerate a large amount of electrical power. As discussed above, only alimited portion of outer surface 1302 of prior art airship 1300 may becovered with prior art PV cell modules 1308. Furthermore, because priorart PV cell modules 1308 are located solely near top 1310, prior art PVcell modules 1308 will only receive maximum sunlight (and be able togenerate their maximum electric power) when the sun is over top of priorart airship 1300. Consequently, prior art airship 1300 may not be ableto supply sufficient electric power to enable many modern airshipapplications that have an internal antenna and require a relativelylarge amount of electric power.

Because PV cells 100 are at least partially transparent to radiofrequency electromagnetic radiation having one or more predeterminedfrequencies, PV cells 100 may cover a larger portion of an airshiphaving an internal antenna (e.g., antenna 1306) than prior art PV cells.Consequently, PV cells 100 may be used to generate a larger amount ofelectric power on a airship than can be generated using prior art PVcells. Consequently, PV cells 100 may enable modern airship applicationsthat have an internal antenna and require a relatively large amount ofelectric power.

FIG. 14 is a side plan view of airship 1400 having a plurality of radiofrequency transparent PV cells 100 in PV cell modules 1402. As notedabove, outer surface 1302 is curved, therefore, PV cell modules 1402must be flexible. PV cell modules 1402 are tuned to be at leastpartially transparent to one or more predetermined frequencies ofelectromagnetic radiation generated and/or received by antenna 1306located within interior 1304 of airship 1400. Because PV cell modules1402 are at least partially transparent to electromagnetic radiationtransmitted and/or received by antenna 1306, PV cell modules 1402 cancover a greater portion of surface 1302 than can prior art PV cellmodules 1308.

FIG. 14 illustrates an embodiment wherein airship 1400 includes bothradio frequency transparent PV cell modules 1402 and prior art PV cellmodules 1308. In some embodiments, it may be desirable to use prior art(opaque) PV cell modules 1308 in addition to radio frequency transparentPV cell modules 1402 because prior art PV cell modules 1308 may have ahigher efficiency than PV cell modules 1402. Prior art PV cell modules1308 may be used in areas not requiring radio frequency transparency(such as near top 1404 of airship 1400), and PV cell modules 1402 may beused in areas requiring transparency to one or more predeterminedfrequencies of electromagnetic radiation.

Changes may be made in the above methods, systems and structures withoutdeparting from the scope hereof. It should thus be noted that the mattercontained in the above description and/or shown in the accompanyingdrawings should be interpreted as illustrative and not in a limitingsense. The following claims are intended to cover all generic andspecific features described herein, as well as all statements of thescope of the present method, system and structure, which, as a matter oflanguage, might be said to fall therebetween.

What is claimed is:
 1. A radio frequency transparent photovoltaic cell, comprising: a back contact layer formed of an electrically conductive material; at least one first aperture formed in the back contact layer, each first aperture having a first length; at least one second aperture formed in the back contact layer, each second aperture having a second length, the first length at least five times the second length; and at least one photovoltaic cell section disposed on the back contact layer.
 2. The photovoltaic cell of claim 1, wherein at least one second aperture is disposed between two first apertures.
 3. The photovoltaic cell of claim 1, wherein each first aperture and each second aperture have a cross shape.
 4. The photovoltaic cell of claim 1, wherein about 90 percent of a surface area of the back contact layer is electrically conductive.
 5. The photovoltaic cell of claim 1 further comprising: a substrate; and a plurality of photovoltaic cell sections disposed on the back contact layer opposite from the substrate.
 6. The photovoltaic cell of claim 5, wherein at least one of the apertures partially delineates a plurality of photovoltaic cells.
 7. The photovoltaic cell of claim 5, wherein at least two of the photovoltaic cell sections are electrically connected in a parallel configuration.
 8. The photovoltaic cell of claim 5, wherein at least two of the photovoltaic cell sections are electrically connected in a series configuration.
 9. The photovoltaic cell of claim 1, wherein the photovoltaic cell is disposed on an outer surface of an airship having one or more radio frequency antennas disposed in an interior of the airship.
 10. A method of fabricating a photovoltaic cell back contact layer at least partially transparent to radio frequency electromagnetic radiation having at least one predetermined frequency, comprising the steps of: creating an electrically conductive back contact layer having a thickness of at least one skin depth at a lowest predetermined frequency; and creating one or more apertures in the back contact layer, each aperture at least partially transparent to radio frequency electromagnetic radiation having a predetermined frequency.
 11. An airship, comprising: one or more radio frequency antennas disposed in an interior of the airship; and one or more radio frequency transparent photovoltaic cells disposed on an outer surface of the airship.
 12. The airship of claim 11, wherein each photovoltaic cell further comprises: a back contact layer formed of an electrically conductive material; at least one aperture formed in the back contact layer, each aperture having at least two elements; and at least one photovoltaic cell section disposed on the back contact layer.
 13. The airship of claim 11, wherein each photovoltaic cell further comprises: a back contact layer formed of an electrically conductive material; at least one first aperture formed in the back contact layer, each first aperture having a first length; at least one second aperture formed in the back contact layer, each second aperture having a second length, the first length at least five times the second length; and at least one photovoltaic cell section disposed on the back contact layer.
 14. The airship of claim 11 further comprising a microwave transmission and reception subsystem disposed in the interior of the airship.
 15. A method of providing electric power to an airship having one or more radio frequency antennas disposed in an interior of the airship, comprising disposing one or more radio frequency transparent photovoltaic cells on an outer surface of the airship, wherein each photovoltaic cell is at least partially transparent to radio frequency electromagnetic radiation having at least one predetermined frequency. 